Composite soft magnetic thin films are widely used in commercial applications, for instance, in magnetic recording heads. Also, a perpendicular magnetic recording medium usually has a soft magnetic underlayer of about 2000 Å thickness, which is conventionally made with magnetron sputtering. Magnetron sputtering has several advantages over diode sputtering, such as high deposition rate. It is very difficult, however, to use magnetron sputtering for soft magnetic materials because the targets of soft magnetic materials shunt the magnetic flux from the magnets of magnetron cathodes. Therefore, in order to properly operate the magnetron, a magnetic field of more than 150 Oe is required to be applied above and parallel to the target surface. However, when the magnetic field is about 150 Oe, the deposition rate of soft magnetic materials is still very slow. The conventional method for enabling the magnetron function for soft magnetic materials is to reduce the target thickness to minimize the shunting effect of the target. Even though the magnetron can marginally work by this method, the deposition rate is still too low.
In a perpendicular recording medium, the thickness of the soft magnetic layer is about 2000 Å. This thickness is much larger than the thickness of other layers in the perpendicular recording medium. For example, the thickness of the interlayer, recording layer and overcoat are about 50 Å, about 200 Å, and about 40 Å, respectively. The low deposition rate of the soft magnetic underlayer significantly reduces throughput. Also, thin targets significantly increase shutdown time of sputter machines for changing targets, and are not feasible for mass production.
U.S. Pat. No. 6,033,536 (Ichihara) discloses a magnetron sputtering method using a composite sputtering target consisting of a material having a maximum relative magnetic permeability of 50 or more or consisting of a soft magnetic material which contains two or more phases selected from the group consisting of an M-X alloy phase, an M phase, and an X phase in that at least the simple substance phase consisting of the phase with the smaller atomic weight, M or X, is included, with the proviso that M is not equal to X, M is at least one element selected from the group consisting of Fe, Co and Ni, and X is at least one element selected from the group consisting of Fe, Al, Si, Ta, Zr, Nb, Hf and Ti. In particular, in column 11, lines 19 and 20, Ichihara discloses forming a NiFe film by using a target consisting of a NiFe alloy phase and a Fe phase.
While Ichihara discloses a composite sputtering target (see FIG. 8 of Ichihara) and a magnetron sputtering method, Ichihara is totally different from this invention. Ichihara concerns mainly with the composition consistence of the deposited films. The criterion for the composition of the composite sputtering target of Ichihara is based on atomic weight of the constituent materials (phases) of the target, not the saturation magnetization (Ms) of the constituent materials with respect to the saturation magnetization of the materials with identical composition as that of the resulting composite soft magnetic films. In particular, Ichihara requires that at least one phase of the target must be a simple substance phase having a smallest atomic weight relative to the atomic weights of M and X. Therefore, for forming a NiFe film, Ichihara uses a NiFe alloy phase (M=Ni and X=Fe) and a Fe phase (X=Fe), wherein the simple substance phase, i.e., Fe, has the smallest atomic weight relative to Ni and Fe. However, according to Ichihara, the simple substance phase having the smallest atomic weight, e.g., Fe, has a higher saturation magnetization than that of the sputter deposited film, e.g., NiFe. It is, therefore, more difficult to sputter-deposit Fe by magnetron than FeNi, when saturation magnetization is the main concern.
There are several problems in Ichihara's method, which need to be solved. For example, the erosion of the target of FIG. 8 of Ichihara will not be uniform during sputtering because the magnetic field along the circumferential direction above this target surface will not be uniform, but will change dramatically. The field will be very strong above the Zr phase, and weak above the Fe and FeZr phases, resulting in non-uniform erosion of this target in the circumferential direction.
The soft magnetic target materials do not operate at the maximum permeability regime for a magnetron sputtering application. The magnetic field above the target surface in the plasma and parallel to the target surface must be greater than about 150 Oe to enable magnetron sputtering. The magnetic fields inside and outside the target surface at an area near the target surfaces and parallel to the surfaces of a target are nearly identical. Therefore, the magnetic field inside the target would be about 150 Oe or more. The highest magnetic induction of the widely used soft magnetic materials is known to be about 24000 Gauss. When B=24000 Gauss and H=150 Oe, μ=160 because B=μH, where B is magnetic induction, μ is permeability, and H is magnetic field. This value of permeability is 2 to 3 orders of magnitude lower than the maximum permeability of most of commercial metallic soft magnetic materials. Therefore, maximum permeability of the target materials is not a concern of this invention. Instead, this invention is concerned with the problem of high saturation magnetization of the target materials, which is the cause of low pass through flux above the target surface.
Despite some advances in magnetron sputtering of soft magnetic films, there still is a need to find a method that can be used for efficient production of soft magnetic films with single-phase and substantially uniform composition by magnetron sputtering.